Consider first, that under this humble similitude of so small a thing as a grain of mustard see, great and divine truths are delivered to us by truth itself, when he tells us that the kingdom of heaven is like unto a grain of mustard seed. The kingdom of heaven, in the gospel, is taken in three different ways; sometimes for God’s eternal kingdom, to which the just are invited, Matt. xxv. 34, ‘Come, ye blessed of my Father, possess the kingdom prepared for you,’ & c., of which also it is said, Matt. xiii. 43, ‘The just shall shine, as the sun, in the kingdom of their Father.’ At other times the kingdom of heaven is taken for the church of Christ, in which he reigns for ever, as in his kingdom; and the institution and intention of which is to bring men to heaven: and thus the kingdom of heaven is likened to ‘a net cast into the sea, and gathering together of all kinds of fishes,’ & c., Matt xiii. 47; ‘and to ten virgins who went out with their lamps to meet the bridegroom,’ Matt. xxv. & c.: and of this kingdom it is said, that our Lord shall send his angels, (at the end of the world,) and ‘they shall gather out of his kingdom all scandals, and them that work iniquity; and shall cast them into the furnace of fire,’ & c. At other times again the kingdom of heaven is taken for the kingdom by which God reigns, by faith, grace, and love in the soul of good Christians: and thus ‘the kingdom of heaven is likened to a treasure hidden in a field;’ and to ‘a pearl of great price,’ Matt. xiii. 44, 46; and of this kingdom it is said, Luke xvii. 21, ‘Lo, the kingdom of God is within you.’ Now, the kingdom of heaven, according to all these three acceptations, is likened to a little grain of mustard seed; because all our good, faith itself, grace, and all our happiness, both for time and eternity, is grounded on humility. We must be little and humble upon earth; and we must become ‘as little children, or we shall never enter into the kingdom of heaven,’ Matt. xviii. 3.
Consider 2ndly, how well this similitude agrees to this spiritual kingdom of Christ in his church. Take a view of the beginning of this kingdom of heaven in a few poor fishermen, utterly destitute of any one of those advantages that might recommend them according to the world; see its very founder himself, a poor man, rejected, condemned, and put to a most disgraceful death, by public authority, at the unanimous desire of both the senate and the people of his own nation: then observe the most fundamental principles and practices upon which this kingdom was first founded and established: its doctrines most shocking to human pride; its maxims and precepts most insupportable to the natural inclinations of flesh and blood: and you shall find in all this the resemblance of the mustard seed; small, mean, inconsiderable, and contemptible in the eyes of the world. But then observe how quickly this little grain, after it had been buried, as it were, in the earth, sprung up, and even grew up into a large tree, which spread it branches far and near, by the wonderful progress of the church and kingdom of Christ made in short time over all the earth; see the many thousands of martyrs and other saints, of all states and conditions, it quickly produces; with innumerable examples of the most heroic virtues, such as none of the schools or sects of the philosophers, or any of the ancient or modern sages of the world, with all their learning and eloquence, and all their pretensions to wisdom, could ever come up to. And in all this admire and adore the wonderful ways of God, who ever delights in showing forth his greatness in things that are little; and in choosing the foolish things of the world and such as are weak, mean, and contemptible in the eyes of the world, to be the instruments of his greatest works.
Consider 3rdly, that this grain of mustard seed is also very expressive of the kingdom of God, by which he reigns by grace in our souls. The beginnings of this kingdom are small, like the mustard seed; the very first foundation of it must be laid by humility, of which the mustard seed is the emblem: for a contrite and humble heart is the most essential ingredient of the conversion of the soul to God, without which the kingdom of divine grace can never be established in the soul. Then this divine grace, like the grain of mustard seed, before it can spring up and produce the tree of Christian perfection, must first be sown, and as it were buried in the earth, by letting it sink deep into the soul and by harbouring it there, by the means of serious and frequent meditations, and the practice of mental prayer. For it is thus only that the soul can be qualified to grow up in all Christian virtues, till she become herself the kingdom of God, and a kind of heaven upon earth; the very temple in which God chooses to dwell; the house of God and the house of prayer. And thus the little grain of seed will grow into a great tree.
Conclude with a serious resolution to seek henceforward in good earnest this kingdom of heaven, represented by the mustard seed; which, as thou here seest, is not out of thy reach, since it may be found here upon earth, and that too without going any farther to seek it than into thy own interior; where, if thou properly seek it, by recollection and mental prayer, thou shalt quickly come at it, and be put in the possession of it. And all good things shall come to thee together with it.